Is the final Super Saiyan stage in the Dragon Ball GT anime
Super Saiyan 4 is a Saiyan form present only in the anime sequel Dragon Ball GT, and it has never been mentioned or hinted in the manga at all. The only way a Saiyan can attain Super Saiyan 4 is if he is already a Super Saiyan, can transform into a Golden Great Ape and regain conscious control over the form. This transformation is the ultimate combination of both the humanoid Super Saiyan and the Golden Great Ape. After this form is attained once, it can be accessed at any point. It is also hinted that only pure-blooded Saiyans can achieve this final and ultimate Super Saiyan transformation, as Baby had full consciousness while as a Golden Great Ape, but he still did not transform. However, Baby was a parasite, possibly halting the final evolution. It is unknown if Gohan or any of the other Human/Saiyan hybrids could ever reach this form, as they too had tails at birth. This form was first achieved by Goku when battling Baby Vegeta. Somehow, the Elder Kai already knew of Super Saiyan 4 and its abilities.
In this form, the Saiyan's hair returns to its original color, the tail regrows and the body is covered in a shade of red fur. A Saiyan in this form also possesses a shadow trim around the eyes and over the eyelids that varies in color. The hair is lengthened, but not nearly as long as Super Saiyan 3. The aura in this form loses the arcs of electricity, instead becoming pure fire. However, in the case of the Fusion warrior Gogeta, the coloration of the aforementioned features seemingly switch, as Gogeta's hair became a bright shade of red whereas his fur became a dark shade of brown and the trim around his eyes was black instead of red. The great power and speed enhancements brought on by this form easily make it the most powerful Super Saiyan form in the series. For example, while Super Saiyan 3 Goku was almost unable to even phase Baby Vegeta, as soon as Goku transformed into a Super Saiyan 4, their roles in power became polar opposites, with Baby Vegeta being almost unable to hurt Goku at all. The users of this Super Saiyan transformation have signature techniques that are only usable in this state. For example, Goku develops an attack he calls " 10x Kamehameha", as it is ten times (10x) stronger than the average Kamehameha Wave, while Vegeta, in his Super Saiyan 4 state, develops an attack he calls the " Final Shine Attack", which is much greater than his Final Flash attack. Unlike the Super Saiyan 3's strain due to the utilization in ki, this form does not require as much energy consumption as Super Saiyan 3's, allowing the user to stay in the state at a much longer duration.
Throughout mid Dragon Ball GT (after the Baby Saga), Super Saiyan 4 became Goku's weapon of choice as the villains became even stronger. After being pushed to the limit by Baby, Golden Great Ape Goku transformed into a Super Saiyan 4 after Pan calmed him down. In this form, Goku completely dominated Baby (none of the villain's attacks could even hurt him) and eventually finished him off with a 10x Kamehameha wave.
Goku's next opponent would be Super Android 17, who proved to be Super Saiyan 4's first real challenge. Because of his energy absorbing abilities, Super 17 was able to turn all of Goku's energy attacks into his own power, making him stronger with each attack. Eventually, Goku reverted to his base form and killed the android (with the assistance of Android 18) using his Dragon Fist attack followed by a Kamehameha.
Super Saiyan 4 proved to be incredibly helpful when Goku used it to take down most of the evil Shadow Dragons. However, he met his match in Syn Shenron, the strongest of the seven dragons, who had absorbed all of the Dragon Balls becoming Omega Shenron. Omega's power proved to be far greater than that of a Super Saiyan 4's, and Goku was thrashed around for most of the fight. Seeing how bad this situation had become, Vegeta used Bulma's Blutz Wave generator to grow a tail without having to wait for it grow naturally. With a tail Vegeta was able to become a Golden Ape like Goku, and then when he finally gained control, he was successful in becoming a Super Saiyan 4. However, even as a team, Goku and Vegeta still stood no chance against the dragon. It wasn't until they fused into Super Saiyan 4 Gogeta that their power finally surpassed Omega Shenron's. However, Gogeta spent too much time playing around with Omega Shenron and the fusion wore off in minutes due to his own tremendous power, and he ultimately failed to finish him off in time. Goku eventually killed Omega Shenron with the Universal Spirit Bomb. It is unknown if, with proper training, Goku and Vegeta could have become powerful enough to match Omega Shenron without needing to fuse, as the saga ends with Omega's defeat and Goku leaving with Shenron.